1. Field of the Invention
The present invention relates to an image forming apparatus employing a fixing unit for fixing a toner image transferred onto a recording medium (e.g., transfer sheet).
2. Description of the Background Art
Typically, image forming apparatuses using electrophotography employ a fixing unit using heat and pressure to fix images on recording media. The fixing unit includes a fixing belt having a heat source therein and a pressure roller, with the fixing belt and the pressure roller forming a pressing portion (or nip portion) therebetween. When a transfer sheet having an un-fixed toner image thereon passes the nip portion, the fixing belt and the pressure roller apply heat and pressure to the un-fixed toner image to fix the toner image on the transfer sheet.
Such image forming apparatuses may use various sizes of transfer sheets such as A4, A3, or the like as a recording medium. However, the fixing belt has a given belt width, and accordingly, continuous image formation on transfer sheets sized narrower than the fixing belt can result in an uneven heat distribution between a center portion and edge portions of the fixing belt. This uneven heat distribution arises because, in the fixing unit, the fixing belt is heated by the heat source, by which the fixing belt receives heat energy. If small-sized transfer sheets are used, such transfer sheets may pass the center portion of the fixing belt but not the edge portions of the fixing belt. If such condition occurs, heat energy at the center portion of fixing belt is consumed but heat energy at the edge portions of the fixing belt is not consumed, by which temperature increases significantly at the edge portions of the fixing belt. Such significant temperature increase at the edge portion of fixing belt may accelerate deterioration of a surface layer of the fixing belt and of the pressure roller, ultimately resulting in defective images.
Further, in such fixing unit, heat energy may not be effectively and efficiently used because heat energy of the heat source is supplied to an area that a transfer sheet is passing (referred to as sheet-pass area), and also supplied to an area that a transfer sheet is not passing through (referred to as sheet-not-pass area).
In light of such heat energy issue, JP-2006-267420-A discusses another type of fixing unit having a heat roller and a pressure roller. The heat roller includes a heat source (e.g., halogen lamp), a rotatable light-shield member having a cylindrical shape that encloses the halogen lamp, a fixed sleeve disposed outside of the rotatable light-shield member, and a rotatable sleeve disposed outside of the fixed sleeve. The fixed sleeve has a rectangular-shaped slit extending in an axial direction (or width direction) of the heat roller, and the rotatable light-shield member has a triangular-shaped slit having one side extended in the axial direction of heat roller.
With such a configuration, a window can be set by aligning the rectangular-shaped slit and triangular-shaped slit by rotating the rotatable light-shield member to a given angle. A size of the window may be adjusted in view of the sheet width of the transfer sheet. Light emitted from the halogen lamp passes through the adjustable window and irradiates an inner face of the rotatable sleeve, which is a heating area (or sheet-pass area).
However, in such fixing unit, the fixed sleeve and the rotatable light-shield member are interposed between the heat source and the rotatable sleeve. Accordingly, much of the heat energy of the heat source may be absorbed by the fixed sleeve and the rotatable light-shield member, and thereby heat energy may not be effectively and efficiently used to heat the rotatable sleeve.
Further, a height of the rectangular-shaped slit in a sheet transport direction is set smaller than a width of the triangular-shaped slit in the sheet transport direction to adjust the size of the window in a sheet width direction. Accordingly, there is a limit on the size of the window in the sheet transport direction, which is perpendicular to the sheet width direction.
Further, because the size of the above-mentioned window in the sheet transport direction is limited, heat energy to heat the rotatable sleeve may need to be increased by using a heat source having a larger heat generating capacity. However, such larger capacity may unfavorably increase both the size of the fixing unit and an energy consumption level.